Processes for preparing carbonyl compounds by gas-phase oxidation over copper or silver catalysts in the presence of volatile phosphorus compounds are known from the prior art.
Thus, EP-A 007 570 describes a process for preparing glyoxal by gas-phase oxidation of ethylene glycol by oxygen over a copper-containing oxidation catalyst in the presence of phosphorus compounds which are volatile under the reaction conditions, in which the amount of phosphorus of from 1 to 100 ppm, based on ethylene glycol used, is added with the starting compounds. These processes give unsatisfactory glyoxal yields of up to 70 mol %, based on ethylene glycol reacted.
According to the processes of U.S. Pat. No. 4,282,374 and U.S. Pat. No. 4,503,261, advantageous results in respect of the life of the catalysts and the glyoxal yield are obtained in the gas-phase oxidation of ethylene glycol over copper catalysts or over a layer catalyst of copper and silver crystals if the reaction is carried out in the presence of a volatile phosphorus compound, with the amount of phosphorus (calculated as P) being from 1 to 100 ppm or from 0.5 to 20 ppm, based on the weight of ethylene glycol, and the phosphorus being introduced together with the starting compounds upstream of the catalyst bed. However, in these processes it has been found that the glyoxal yield and product purity become increasingly worse with time on prolonged operation. This disadvantage is attributable to increased formation of formaldehyde and of CO/CO.sub.2.
EP-B 0 271 812 proposes, for the preparation of carbonyl compounds such as glyoxal, a gas-phase oxidation of alcohols using an oxygen-containing gas in the presence of copper- or silver-containing catalysts and a phosphorus compound which is volatile under the reaction conditions, in which the phosphorus compound is mixed in one portion of less than 0.5 ppm, based on the weight of alcohol used and calculated as phosphorus, into the gaseous starting mixture prior to the reaction over the catalyst.
According to the process described in EP-B 0 271 812, glyoxal is obtained in yields of up to 80 mol %.
The above processes of the prior art have the disadvantage of an unsatisfactory yield. In the known processes, glyoxal is obtained as an aqueous solution contaminated with glycol aldehyde, formaldehyde and organic acids. Further undesired by-products are the combustion products CO, CO.sub.2 and H.sub.2 O which are formed. As a result of the by-products, the known processes have the additional disadvantage of unsatisfactory catalyst operating lives.
In addition, for many applications the presence of formaldehyde in the glyoxal is highly undesirable because of the toxicological properties and the high reactivity of the formaldehyde. Since formaldehyde can be removed from the crude glyoxal only at considerable expense and with the acceptance of yield losses, for example by treatment with steam or by chemical reaction, it would be useful to have a process which allows glyoxal to be prepared by catalytic gas-phase oxidation of ethylene glycol even at long operating times while largely avoiding the formation of interfering by-products.